The reduction of water consumption and use is emerging as a top priority for all types of power plants as a result of water supply constraints in many regions of the world. Several factors contribute to intensified water scarcity, including increased demands for electricity, increased water use in other sectors (for example, agriculture, municipal water supply, mining, and manufacturing), tightened government regulations, population growth, new development, and weather variation (including precipitation and temperature). Constraints on cooling water supplies impact plant site and permitting decisions and current plant operations. Furthermore, there is increasing pressure on the industry to eliminate once-through cooling systems. However, replacing once through systems with the more common wet cooling towers may reduce the water circulation rate but does not reduce the overall water consumption (i.e., same amount of water is evaporated in the cooling process).
More specifically, utility scale thermal power plants traditionally have used water from an evaporative cooling tower to condense the steam coming from the low temperature exhaust of a condensing steam turbine used in the thermal bottoming cycle. This results in large amounts of water evaporated for every MW-Hr of power produced. Moreover, such water usage is common in many thermal power plants regardless of whether the thermal energy source is from a coal fired power plant, a nuclear power plant, or a gas turbine, combined cycle power plant. It has been estimated that the approximate water consumption of each type of power plant is:
Combined Cycle=210 gal/MWH
Nuclear=820 gal/MWH
Coal=760 gal/MWH
Biofuel & Concentrated Solar Power=720 gal/MWH
Recently combined cycle plants have been designed to replace the water-cooled surface condenser and the evaporative cooling tower with a large air-cooled steam condenser to operate in conjunction with the condensing steam turbine. Large air-cooled condensers can condense the low pressure exhaust from the condensing steam turbine directly in the air-cooled steam condenser. It is estimated that approx 70 power plants in the U.S. have installed some type of air-cooled steam condenser on the steam bottoming cycle of combined cycle power plants. This use of air-cooled steam condensers has eliminated the traditional circulating cooling water loop and the cooling tower, and is an effective way of eliminating the use of water for cooling in the thermal bottoming cycle of a combined cycle plant. However, various drawbacks to prior art air-cooled steam condensers exist. First, to maximize the power output from the steam turbine, the condensing temperature and pressure of the steam must be as low as possible. Thus, typically, the condenser is operated below atmospheric pressure. However, operating the condenser below atmospheric pressure can lead to air infiltration which can lower the capacity output of the power plant and increase corrosion of power plant equipment, which increases the maintenance requirements of the boiler feedwater system. In addition, because steam has a comparatively large volume at low pressure, the back end sections of the steam turbine must be sized quite large for the large specific volume required for this very low pressure steam, thereby adding to the expense and complexity of the overall system. As an example, each pound of steam requires 333 cubic feet at 1 psia (102° F. condensing temperature) or 255 cubic feet at 2.2 psia (120° F. condensing temperature) which are typical operating ranges for an air-cooled condenser. Since the latent heat of vaporization of steam is about 1025 BTU/lb at 120° F., it requires a volume of (255 cubic feet/lb×lb/1025 BTU=) 4 cubic feet/BTU. For this reason, the size of the headers, distribution pipes and tubes in an air-cooled condenser must be relatively large to accommodate the very large volumes of low pressure steam in the system. One result is added capital cost. Moreover, traditional air-cooled steam condensers are typically very large, A-frame designs with fans forcing ambient air up through the A-frame arranged condenser coils. A-frame systems such as this are necessary in order to adequately drain the condenser coils of the steam condensate due to the very low steam pressures. However, such A-frame designs impose added fan power requirements and do not represent ideal fan airflow across the coils, thereby inhibiting the effectiveness of the air-cooled condenser in the efficiency of the overall power system.
Another disadvantage of the prior art practice of utilizing air-cooled steam condensers for the bottoming cycle of combined cycle thermal power plants is that the output of the air-cooled combined cycle plant will be degraded more than that of the traditional water-cooled combined cycle plant because the air-cooled condenser rejects its heat to the higher dry bulb temperature rather than the colder wet bulb temperature of a water cooled combined cycle. This degradation occurs at all temperatures, but especially during the high ambient temperature periods. As the ambient temperature rises, the output from the steam turbine will be reduced due the higher backpressure caused by the higher condensing temperatures experienced by the air-cooled condenser, especially during hot periods of the day when the heat from the condensing steam must be transferred to the ambient air temperature. Also as the ambient temperature increases, the difference between the dry bulb temperature and the coincident wet bulb temperature tends to increase, thereby causing a corresponding increased reduction in the air-cooled combined cycle plant output versus the output of a water cooled plant. This reduction in both gas turbine and steam turbine output occurs generally during the time of peak stress on the electrical grid—a time when power demand is usually highest due to peak HVAC loads, yet when the ability of the gas turbine generation fleet capacity is usually at its lowest. For the foregoing reasons, power plant design continues to strongly favor the more water consumptive wet cooling towers for combined cycle plants rather than air-cooled condensing.
There is an increasing need for new designs which can minimize water usage in both existing as well as future new power plants and yet maintain plant power output, especially during high ambient temperature peak periods.